1. Field of the Invention
The present invention relates generally to laminated parts. More particularly, the present invention relates to lamination stacks formed by stamping a plurality of lamination layers from a sheet of stock material and the methods and apparatus, i.e., progressive dies, used in the manufacture of such laminated parts.
2. Description of the Related Art
The manufacture of parts, e.g., stators and rotors for electric motors, employing stacked laminas is well known in the art. Typically, the laminas are blanked from a continuous strip stock and then stacked and bound together to form the completed part. Progressive die assemblies for producing laminated stacks wherein a strip of lamination material is fed through a sequence of punching steps to progressively form the laminas to the desired end configuration are also well known.
It is also known to form, in the laminas, interlock tabs which extend below the generally planar lamina surface and engage slots formed in the next lower lamina. In this manner, a plurality of laminas may be stamped from a single sheet of strip stock and formed into an interconnected lamina stack in the die by means of interlocking tabs and slots. More specifically, to form an interconnected lamina stack each lamina, except the bottom lamina of the stack, may have a plurality of arcuately spaced interlock tabs (typically ranging from 3 to 8 circumferentially disposed tabs) depressed from the lamina lower surface adjacent to slots formed in the next lower lamina. Each interlock tab engages a corresponding slot in the next lower lamina of the stack, generally by the entire thickness of the tab. The bottom lamina of the stack may have the interlock tabs blanked and removed to avoid interlocking the bottom lamina with the next lower lamina which forms the top lamina of the previous stack. In rare instances the tab may lock as deeply as two lamina thicknesses, in which case two end laminations must be blanked.
Rotor laminas generally include a plurality of skewed conductor slots which are formed around the periphery of the rotor stack in arcuately spaced relation to one another by rotationally indexing the laminas with respect to the rotor stack. Indexing involves rotating the rotor stack and the last produced lamina relative to each other by a predetermined rotational increment so that, when the laminas are combined in a stack, the rotor conductor bar slot defined by adjacent conductor slots are skewed or slanted relative to the stack axis. Stator stacks, on the other hand, include winding slots around the inner periphery of the stack which extend parallel to the stack axis, without skew, and are shaped to receive the stator windings. In some circumstances, however, it may be desired to build an "inside-out" motor wherein the outer lamination stack forms the rotor and would, thus, require skewed slots.
Another system of forming a stack involves loosely stacking the laminas as they are formed and blanked from the stock material in a progressive die assembly. After all the laminas for a given stack are collected, they are shuttled to a pressing station and the laminas are pressed together to engage the interlock tabs and thereby form the lamina stack. Loosely stacking the laminas after they are blanked from strip stock has several disadvantages; loose stacking and subsequent pressing does not as consistently lock adjacent laminas together; the required handling slows production times; and the system lacks a means for automatically correcting thickness inconsistencies of the stock material or creating a desired skew angle for the conductor slots. A similar process can be employed without the use of interlocking features on the laminas. Assembly of the non-interlocked laminas requires the welding, keying or riveting (or pinning) of the laminas to interconnect the laminas in a stack.
In response to these problems, an autorotation system for compensating for the nonuniform stock thickness was developed which both rotates and interlocks the stacked laminas. This system compensates for variations in lamina thickness while still properly skewing the conductor slots of rotor laminas, as described in U.S. Pat. Nos. 4,619,028; 4,738,020; 5,087,849 and 5,123,155, all assigned to the assignee of the present invention and the disclosures of which are incorporated herein by reference. In the system disclosed in the aforementioned patents, the choke barrel holding the lamination stack is automatically rotated before each lamina is blanked from the strip stock and the lamina's circumferentially disposed tabs are interlocked with the slots of the uppermost lamina of the incomplete lamination stack within the barrel.
In the apparatus and method disclosed in the aforementioned patents, the individual laminas are typically rotated through an angle of 180.degree.. Although the laminas may be rotated through other angles, the angle must be at least 360.degree./(number of interlock tabs) so that the interlocking tabs and slots are properly aligned.
The above described improvements have been implemented with rotor laminations and stator laminations which have identical outer perimeters which enables their insertion into a choke barrel designed to hold a lamination having the outer perimeter configuration of the laminations being stacked. Many of these improvements require the use of interlock tabs in combination with autorotation of a partially formed lamina stack.
Autorotation requires the use of a rotating choke barrel which firmly holds the partially formed lamina stack in position as blanked laminas are forced into engagement with the uppermost lamina of the stack. The choke barrel is typically configured to match the outer perimeter of the blanked lamina and may be slightly undersized, e.g., by 0.001 inch, so that the laminas will be firmly held and accurately positioned within the choke barrel. The laminas located in the choke barrel thereby provide a back pressure or resistance which facilitates the entry of the interlock tabs of the next lamina when it is pressed into the choke barrel.
In certain applications, however, it is desirable to have a lamination stack, typically a stator core but also rotor cores in some situations, wherein some of the laminations have an outside perimeter which differs in shape and/or size from the remainder of the stack of laminations, i.e., the laminations in the stack have a plurality of distinguishable configurations. For example, the stator core may incorporate a fastening feature, such as a projecting flange, to provide a mounting surface which is integral with the stator core, or the stator may incorporate a sealing feature to provide a seal between the housing of the motor and the stator core for motors to be used in environments which include flammable vapors. To incorporate such features, a fraction of the laminations in a stack are manufactured with integral portions which provide such features.
Traditionally, the manner in which stator cores having a plurality of outer perimeter configurations have been produced is to stamp the differently configured laminas in separate dies, i.e., each die provides only a single lamina configuration. The plurality of dies produce loose laminations having the desired plurality of outer perimeter configurations. The laminations must then be manually assembled at a station where laminations of the different outer perimeter configurations are placed in the proper vertical stack arrangement and are pressed together to interlock the laminas. Instead of using interlocking tabs, the laminas may also be secured together in some other conventional fashion such as by the use of clamps, pins, rivets or welds.
There are several drawbacks to this manner of manufacturing a lamination core having laminations with a plurality of outer perimeter configurations. For one, the manufacturing process is relatively expensive due to the use of multiple dies and the large amount of labor and handling which is required. Additionally, the process does not allow for the automatic correction of lamina thickness inconsistencies.
Another problem with this method of manufacture is that it often produces stator cores having winding slots with slight discontinuities and sharp edges. Because separate dies are used to form the differently configured laminas, the stator winding slots are punched by different dies. Although similar in shape, the different punches cannot be precisely identical and will generally have minor inconsistencies which, when the differing laminas are stacked, cause the slots in adjacent laminations to misalign, thereby creating slight discontinuities and sharp edges in the winding slots at the points where the two differently configured laminas meet. These small discontinuities can scratch and damage the winding coil wires which are inserted into the winding slots.
The discontinuities of the projections which define the winding slots and interior surface of the stator core also reduce the efficiency of the electric motor or generator which is produced with the stator core. The efficiency of the motor or generator may be reduced if the gap between the stator core and rotor core is enlarged to account for the discontinuities present on the interior surfaces of the stator core because the efficiency of the motor or generator is decreased as the gap increases.
The manufacture of lamina stacks wherein individual laminas are comprised of two or more discrete segments also presents significant manufacturing difficulties. It is often impractical to manufacture lamina stacks wherein one or more of the laminas is formed by at least two discrete lamina segments. Laminas comprised of a plurality of discrete segments present difficulties in maintaining the proper alignment between the various lamina segments which comprise the individual lamina and between the lamina segments and the other laminas which comprise the remainder of the lamina stack.
Thus, what is needed is an apparatus and method for producing lamina stacks which include laminas comprised of a plurality of discrete lamina segments and laminas with a plurality of differently configured outer perimeters.